The invention relates to a method of operating an internal combustion engine, in particular a compression- or auto-ignition internal combustion engine with direct fuel injection including a cylinder with a cylinder head having intake and exhaust valves and a piston for compressing combustion gases in the cylinder into which fuel is injected for combustion therein.
In direct-injection internal combustion engines with auto-ignition, lean homogeneous fuel/air mixtures are often made to auto-ignite so that high efficiency levels and improved exhaust emissions are obtained. In such internal combustion engines which are referred to as HCCI or PCCI internal combustion engines, that is, internal combustion engines with spatial compression ignition, a lean basic mixture of air, fuel and retained exhaust gas is generally formed at partial load and auto-ignited. During full load operation, a stoichiometric mixture is frequently formed and spark-ignited because at high loads steep rises in pressure could occur in the combustion chamber due to the auto-ignition and these would adversely affect the operation.
Patent DE 198 10 935 C2 discloses a method of operating an internal combustion engine which operates according to the four stroke principle and in which a homogeneous lean mixture of air, fuel and retained exhaust gas is formed and is burnt after compression ignition. In this context, there is an intermediate activation phase in order to expand the operating range of the motor with compression ignition: During the compression of the retained exhaust gas, an activation fuel quantity is injected into the combustion chamber and distributed as homogeneously as possible in the combustion chamber with the residual components of the mixture. In this way thermal energy is supplied to the mixture by power and compression so that a chemical reaction or ignition is initiated in the vicinity of the top dead center of the charge cycle. The ignition time of the fresh charge can be controlled during the main compression by the timing and the quantity of the activation injection.
In accordance with the current state of the art however, it is difficult to selectively control the combustion described above since the time of auto-ignition depends very greatly on the parameters of the engine and the ambient conditions. For this reason, attempts are made to control the initiation of the compression ignition using suitable control variables, for example by means of a cylinder pressure signal. Such concepts are, however, associated with a high degree of expenditure on engine control technology which results in higher manufacturing costs of such internal combustion engines.
It is therefore the object of the present invention to provide a method of operating an internal combustion engine in such a way that the timing of the auto-ignition can be more reliably controlled.